The age of the Bitterroot metamorphic core complex, situated along the Montana-Idaho border, has been debated for many years. Some researchers assign a Late Cretaceous age to extension in the area, relating unroofing of the metamorphic core to gravitational sliding off the Idaho batholith, whereas others believe that the extension was a Cenozoic phenomenon. One of the largest and most important extensional structures in the Bitterroot complex is the Bitterroot mylonite zone. An undeformed rhyolitic dike that transects mylonitic fabrics in the southern part of the zone yields a 40Ar/39Ar K-feldspar age of 46.4 ± 0.8 Ma, establishing a minimum age for the structure. Previously published U-Pb data for deformed granitoids from other parts of the shear zone suggest that the maximum age of mylonitization is 48 ± 1 Ma. Documentation of 46-48 Ma extension in the Bitterroot complex suggests simultaneous deformation in the Rocky Mountains Basin and Range and Omineca extensional provinces and reinforces the idea that the Lewis and Clark fault system may have acted as an extensional transfer zone in middle Eocene time.